1. Field of the Invention
This invention relates to the field of focusing devices for cathode ray tubes, and in particular, to an electromagnetic focusing device utilizing permanent magnets.
2. Description of Related Art
Two general types of magnetic focusing devices are now used. One type of focusing device uses a winding for generating the magnetic field, often referred to as dynamic focusing. The other type of focusing device combines permanent magnets and adjustment windings. These two systems have advantages and disadvantages:
A typical example of the type of focusing device 8 using a winding is shown in FIG. 1. The magnetic field 10 necessary for focusing the electron beam 12 on a screen 9 is produced by an annular winding or coil 14 enclosed in a frame 16. The annular frame 16 has an annular opening or gap 18 which substantially confines the field 10 within the boundary of the winding 14 and frame 16. The focusing point varies if the magnetic center of the field exhibits any hysteresis. Accordingly, the frame 16 is made from extra pure iron to avoid this hysteresis and assure that the same magnetic field will be generated at each start-up.
Focusing is obtained by adjustment of the direct current in the coil. The power brought into play is high, for example on the order of 10 watts for a cathode ray tube taken as reference and powered at a level of 25 kV. The power requirements, especially in connection with projection tubes, can range as high as about 40 kV. At this voltage level, the same focusing coil on the same tube will require about 16 watts. This power provides only the static focusing. It is often desirable to add a supplementary winding (not shown) to obtain dynamic focusing (focusing in the corners of the tube). Dynamic focusing presents two problems in particular. Firstly, high frequency operation, for example 64 KHz, results in very great energy dissipation. Secondly, there is considerable magnetic coupling with the static winding.
A focusing device having a static magnetic field generator can utilize a toroidal permanent magnet made of a material of high thermal stability. Such a magnet solves the problem of dissipated power in the static winding. The focusing adjustment is made by means of an auxiliary winding, having only a small number of turns. This winding dissipates energy at a power level which is negligible as compared to the energy dissipated by a focusing device with fully dynamic coil. Coupling to the dynamic focusing device is also reduced, due to the smaller number of turns.
Although this system works, it too has some disadvantages. Firstly, the magnetic field is not uniform. The material of which the permanent magnet is made is sintered and is not perfectly homogeneous. This lack of homogeneity leads to magnetic field anomalies which create poles. These poles cause spot deformation to appear, such as astigmatism which can result from 4 poles, and coma which can result from 6 poles. The permanent magnet can be implemented as a plurality of discrete permanent magnets, as shown in U.S. Pat. No. 4,758,762. Eight bar magnets, each surrounded by a coil, are disposed in a radial, coplanar array for generating a static focusing field.
Secondly, is the problem of adapting the system to different tubes. The power of the focusing device is proportional to the magnetic mass of the magnet. Therefore, as the magnet must be magnetized to saturation in order to avoid any risk of demagnetization, it is suitable only for a value of high acceleration voltage of the cathode tube. In fact, the required power of the focusing device varies with the acceleration voltage of the tube. This makes it necessary to have several types of magnets for adapting to various tubes, which results in high tooling costs.